Properties of piezoelectric AlN layers deposited by reactive pulse magnetron sputtering

Reactive pulse magnetron sputtering of Al targets in a gas mixture of Argon and Nitrogen allows the deposition of AlN layers at high deposition rates of up to 200 nm/min. In the reported experiments films were deposited AlN onto unheated substrates with a thickness of typically 10 ?m. Deposited films have been characterized for a variety of layer properties using e.g. XRD, SEM, profilometry, weighting and piezoelectric measurements regarding crystalline structure and orientation, surface morphology, density, film stress and piezoelectric coefficient d33. The characterized AlN films can be classified into 2 groups. The first group shows a nearly pure 001 orientation of the crystalline structure, an undisturbed surface morphology, a high density and a very high piezoelectric coefficient d33 of up to 7.2 pm/V on silicon substrate. The second group exhibits a dominating but not pure 001 orientation, disturbances in the surface morphology, a slightly lower density and a piezoelectric coefficient close to zero. The range of the process parameters pulse mode, pressure, sputtering power and reactive working point to achieve the layers of the first group is very narrow. Surprisingly films with high piezoelectric constant can be obtained both by strong and moderate particle bombardment during deposition using adapted parameter sets. The suitability for the intended application in high frequency ultrasonic phased array sensors systems is investigated using pulse echo measurements.